1. Field of the Invention
The present invention relates to a band rejection filter with attenuation poles that is provided with two or more resonant circuits.
2. Description of Related Art
Conventionally, band rejection filters provided with two or more resonant circuits are known (for example, refer to “Microwave Filters, Impedance-Matching Networks, and Coupling Structures”, G. Matthaei, et al., Artech House Publishers, 1980, pp. 735). In general, a band rejection filter is provided with two or more series resonant circuits 200 connected in series via a transmission line 201, as shown in FIG. 11. The transmission line 201 is the one that has a length equal to the one-quarter wavelength at the resonance frequency of the two or more series resonant circuits 200. The band rejection filter is also provided with an input/output terminal 203 having an impedance Zo.
Since the electric nodes of each series resonant circuit 200 are electrically short-circuited at the resonance frequency of the two or more series resonant circuits 200, the band rejection filter exhibits characteristics of having an infinite attenuation at the resonance frequency of the two or more series resonant circuits 200 and having a limited attenuation in the vicinity of the resonance frequency. A problem with the conventional band rejection filter mentioned above is that in some cases the number of series resonant circuits required to obtain a desired attenuation in a certain frequency band has to be increased and therefore the circuit scale has to be increased. Another problem is that a so-called attenuation pole that provides a maximum attenuation is always formed only at the resonance frequency of the plurality of resonant circuits, and it is therefore difficult to provide a sufficient attenuation in a frequency range of frequencies close to the resonance frequency.